1. Field of the Invention
This invention relates to stand free multi-beamed load cells.
2. Brief Description of the Prior Art
As stated in Strain Gage Based Transducers, 1988 by Measurements Group, Inc., which is incorporated herein as though recited in full, for certain types of applications, the characteristics of the straight cantilever beam can be improved upon. The improvement can be by designs which induce "multiple bending" (reversed curvature) in the beam element. The potential advantages of a beam which is built-in at both ends, and loaded at the center include; intrinsic stiffness and straining line motion of the point of load application as the beam deflects. The spring element also lends itself to relatively easy installation of a full bridge strain gage circuit on the upper surface of the beam. Some degree of non linearity in output can be expected, however, because of the membrane stress produced in the beam (as it deflects) by the rigidly spaced end supports. Additionally, as for most flexural spring elements, it is necessary to vary the section modulus of the beam along its length if the strain gages are to lie in nearly uniform strain fields.
An alternative configuration, a spring element has generally the same bending moment distribution and deflection pattern and retains essentially the same advantages except that the compliance is twice as great if the dimensions are otherwise the same. Because the end restraints are free to move laterally as the upper and lower beams deflect, the membrane stress is eliminated. Any such motion, however, represents a small change in the moment arm of the applied load, which can manifest itself in the form of non-linear response if the ration of the deflection to the beam length is great enough.
Pairs of strain gages are mounted side-by-side on one surface of the beam, or back-to-back on opposite surfaces, to implement a full bridge circuit. The design is sensitive to both the location and direction of the applied load. To function properly, the design must incorporate features to assure that loading can occur only along the intended axis.
A significantly improved form, where the load sensing is accomplished with two beams, joined by relatively massive sections at both ends. With this configuration, externally applied couples are counteracted by axial forces in the sensing beams, minimizing the effects of off-axis loads. One of the drawbacks of the design is its excessive compliance. The deflection which takes place in the beam segments between gage locations not only increases the compliance of the unit, but also degrades the linearity. Better load cell performance can be obtained by either shortening the beams or increasing the beam thickness between gage sites. Such design changes should be made with full consideration of the shear load which must be borne by the element. Strain gage installation and inspection are more difficult when gages are located inside of a hole.
Various forms of the coupled dual-beam arrangement are widely used in load cells for weighing applications.
Another type of bending spring element is the ring. The ring shaped element also has a long, classical history in measurement technology, stemming from the well known Morehouse proving ring, once universally used to calibrate materials testing machines. Although ring type spring elements always involve bending, direct stress is also intrinsic to the configuration, and the combination of the resulting two deformation modes provide the primary distinction from pure beams.
In a basic ring design, the strain distribution in the ring is a complex function of the geometry, and is significantly affected by the design details of the bosses. The bending moment does not vary significantly in the region of the horizontal diameter, the strain distribution is nearly uniform in this area.
The squared ring is easier, less costly to fabricate, decreases the compliance of the spring element, and correspondingly improves the linearity. At the same time, the flexural stiffness at the junctures of the bosses and the ring has been reduced to minimize the sensitivity of the element to off axis load components. There are countless other designs based on the presence of a stress concentrating hole and/or lateral notches in an axially loaded member. A representative configuration, taken from U.S. Pat. No. 3,315,203.
In adapting the ring concept to different load cell specifications for capacity, physical size, etc., the designs sometimes deviate so far from a conventional ring in appearance that their classification as such becomes arguable.
The evolution of beam type load cells has been traced from the basic cantilever beam, through a number of refinements, to a variety of more sophisticated forms with generally superior properties. Multiple beam spring elements are currently very popular, and can be found in many commercial transducers, particularly in low capacity units. It is the need for this last qualifier which leads to the subject matter of the present section.
Although multiple beam designs have good overall characteristics, including linearity and insensitivity to point of load application, they do not lend themselves well to being scaled up for higher load cell capacities. As the capacity of the load cell rises, so does the size of the spring element, along with its mass and, usually, its deflection at rated load. Because of these considerations, spring elements based on the measurement of bending strains are not commonly used in load cells with capacities greater than about 1000 lb. Instead, transducer designers ordinarily turn to one of two other configurations, the shear web or column, to achieve very high capacities in compact, low compliance spring elements.
One of the advantages of the shear web spring elements is its low sensitivity to variations in the point of load application. Static equilibrium considerations decree that the vertical shear force on every section of the beam to the right of the load be the same, and exactly equal to the applied load. Thus the shear in the web should be independent of the point of load application (along the beam centerline), as long as the load is applied to the left of the web. If the strain gages sensed only the shear induced strains, the bridge output would be unaffected by the position of the load or by other bending moments in the vertical plane.
Since the gage grids are necessarily finite in length, however, and thus span a small distance above and below the neutral axis, their outputs are also slightly affected by the bending strains in the web. With the grids centered on the neutral axis, the tensile and compressive bending strains above and below the axis tend to be self canceling in each grid. But the cancellation is usually less than perfect because of small asymmetries in the spring element and strain gage installation.
Because of higher order effects tending to couple the shear and bending strains, it is always preferable to design the beam for the lowest practicable bending moment in the shear web. This would seem to suggest the use of very short beams, but the point of load application must be far enough away from the shear web so that the web behavior approximates the ideal described here. The bending moment at the center of the web is zero, and for a given beam length and rated capacity, the bending moment throughout the beam is halved.
Another type of shear web spring element consists of a metal block in which holes or slots have been machined to form webs subjected to direct shear under axial load. A further example is where the shear webs are produced by drilling a hole longitudinally through the beam. Strain gages oriented at +-45 degrees to the beam axis are then installed inside the hole to sense the shear force, as in U.S. Pat. No. 4,283,941.